1. Field of the Invention
This invention pertains to fabrication of optical holey fibers and to the optical holey fibers made thereby.
2. Description of Related Prior Art
There is significant current interest in the novel optical properties of holey fibers. Holey fibers are typically glass fibers that are fabricated with a network of open channels or holes that run the length of the fiber. Theoretical and computational studies of the properties of holey fibers indicate that these fibers may be superior in many ways to conventional step index or gradient index solid glass fibers. All conventional optical fibers confine light by total internal reflection whereby light is confined within a core that has a higher index of refraction than the surrounding cladding. Holey fibers can confine light using either of two mechanisms: conventional total internal reflection or Bragg diffraction, which is also known as photonic band gap confinement. If light is conveyed through a fiber by total internal reflection, a solid core of higher index of refraction than the clad index is needed. Conveyance of light through a fiber by Bragg diffraction can be achieved with an air core and requires a regular and uniform arrangement of channels through the clad. When conveying light by Bragg diffraction, one gets diffraction for certain wavelengths of light, which requires near-perfect periodicity of the channels for the entire length of the fiber. This periodicity allows light of certain wavelengths to be conveyed through the fiber but will not allow light of other wavelengths, in the stop band, through. These wavelengths will scatter and be lost. For instance, a photonic band gap fiber tuned to green light will allow one to see only green light at the end of the fiber while light of other wavelengths will be lost through the sides of the fiber.
The holey fibers that guide light by conventional total internal reflection are also known as effective index fibers. The characteristics of these fibers include: the core refractive index is greater than the cladding refractive index because the cladding has holes throughout; the bandwidth is broad; and periodicity in the arrangement of the holes is not required. In holey fibers, the cladding refractive index is less than the core refractive index not because the composition of the glass is different but because the cladding glass has holes in it. Thus, the effective index of the cladding is less than that of the core. In holey fibers, the geometry of the hole pattern and the size of the index contrast can lead to novel optical properties, such as endlessly single mode behavior, large effective mode area, anomalous group velocity dispersion, high birefringence, soliton effects, supercontinuum generation and lasing. It is not possible to describe an optimum design of photonic crystal fibers in general since different fibers are optimized for each of the above mentioned properties. Although the optical properties of holey fibers are superior to those of conventional fibers for many applications, conventional fibers are usually preferred over holey fibers. This is due primarily to the inability to fabricate holey fibers uniformly and reproducibly.
Holey fibers that guide light due to Bragg diffraction are referred to as band gap fibers, honeycomb fibers or Bragg fibers. In these holey fibers there is no requirement on the core index to be higher than that of the clad, and can be either be higher or lower than the cladding index, the bandwidth is narrow and the hole arrangement must be periodic and highly uniform along the length of the fiber. Band gap fibers are of particular interest in the telecommunications industry because of the possibility of guiding light with essentially zero loss in an air core. Transmission in an air core virtually eliminates nonlinear effects and significantly reduces scattering. Although holey fibers are of great interest and the physics is quite well understood, experimental studies are hampered due to an inability to fabricate high quality fibers.
The greatest challenge facing designers of holey fibers is fabrication. High quality, uniform holey fibers are extremely difficult to make. They are typically made by first stacking silica glass tubes together into a bundle. The bundle is then fused and drawn in a single step at high temperature. During the fusion and draw process, the interstitial spaces between the tubes should close but at the same time, the channels of the tubes should remain open and the shape of the tubes should not distort. In practice, however, it is found that it is virtually impossible to sufficiently heat the bundle to successfully close the interstitial spaces without causing significant distortion in the circular shape of the tubes and the relative orientation of the tubes. The resulting fibers are irregularly shaped and contain holes in irregular patterns. While this is a problem for the fabrication of all types of holey fibers, it is of particular concern for the fabrication of band gap fibers. Effective confinement of light in a band gap fiber requires a highly periodic arrangement of holes and this periodicity must be maintained along the entire length of the fiber. Non-uniformities in band gap fibers are primarily responsible for the optical losses. In order for the potential of band gap fibers to be realized, new fabrication techniques must be developed that permit the realization of fibers that effectively confine light over kilometers of fiber with acceptably low losses.
Microchannel plate glass and nanochannel glass are microstructured glass materials that are similar in some respects to holey fibers. Channel glasses are porous microstructured glass materials that contain arrays of uniformly spaced, regular channels. Like holey fiber, channel glass is fabricated by first stacking composite glass fibers in a bundle and then fusing and drawing the bundleit to smaller dimensions. The process begins by placing an acid-etchable glass rod into an inert glass tube. This pairing of dissimilar glasses is fused and drawn at elevated temperature into a small rod or fiber of smaller diameter. Several thousand of these composite fibers are cut and stacked in a hexagonal-close-packed arrangement yielding a hexagonal-shaped bundle. This bundle is subsequently fused and drawn at elevated temperature to small rods or fibers. At this stage, the fibers are hexagonal shaped and contain a fine structure of several thousand micron-sized, typically 5 to 10 microns in diameter, acid-etchable glass channels in a hexagonal-close-packed pattern. Commercial microchannel plate glass is made at this point by bundling these fibers together in a twelve-sided bundle and fusing the bundle at elevated temperature. Microchannel plates are obtained by cutting the bundle into wafers and etching in a weak acid solution. Etching removes the acid-etchable glass to yield an array of hollow channels. Alternatively, nanochannel glass may be obtained by stacking the hexagonal shaped fibers into a second bundle. This bundle is fused and drawn in the draw tower. In this manner, glass samples with submicron channels with extremely high channel densities can be achieved. After the last glass draw, the boules are wafered, polished and then etched to remove the acid etchable glass. In this way, a glass with extremely uniform, parallel, hollow channels is obtained.
A significant feature of the fabrication of channel glass, that is primarily responsible for the outstanding uniformity and regularity of the glass channels, is the fact that the softening temperature of the etchable glass is higher than the softening temperature of the matrix glass. Thus, when the composite fibers, or the bundles of composite fibers are heated, the temperature can be carefully adjusted such that the matrix glass flows while the etchable glass retains its shape. The etchable glass rods typically are ground to a precise cylindrical shape before they are used to improve the circular shape of the channels. The flow of the matrix glass is responsible for elimination of the interstitial spaces that are initially present between the stacked fibers of glass. The softening point of the etchable glass is typically about 50° C. greater than that of the matrix glass.
Channel glass can yield glass substrates with channels having aspect ratio (channel length divided by channel diameter) up to approximately 1000. While this is an enormous aspect ratio, it falls far short of that required for holey fibers, where the aspect ratio can be in excess of 109. There is no possibility of obtaining long lengths of holey fibers which have channel aspect ratio that is essentially infinite, following the acid etching techniques used for nanochannel glass.